25 years ago today, 7 May 1990, we published in the IEEE Oakland conference the paper "A Network Security Monitor", which described our design, our "prototype LAN security monitor - hereafter referred to as our Network Security Monitor (NSM)", and results from using this system to monitor our own networks. The next day we published in the DOE Computer Security Group Conference the paper "Network Attacks and an Ethernet-based Network Security Monitor".

From the IEEE paper's "Introduction" section:

Specifically, our goal is to develop monitoring techniques that will enable us to maintain information of normal network activity (including those of the network's individual nodes, their users, their offered services, etc.). The monitor will be capable of observing current network activity, which, when compared with historical behavior, will enable it to detect in real-time possible security violations on the network

That kind of sums up a lot of the work being done today (including by myself again) under names like "cyber analytics" or "security data analytics". Deploying the 1990 version of the NSM we also learned some valuable lessons that shaped future work.

From the "Performance of the N.S.M." section:

The biggest concern was the detection of unusual activity which was not obviously an attack. Often we did not have someone to monitor the actual connection, and we often did not have any supporting evidence to prove or disprove that an attack had occurred. One possible solution would be to save the actual data crossing the connection, so that an exact recording of what had happened would exist. A second solution would be to examine audit trails generated by one of the hosts concerned. Both approaches are currently being examined.

Over the next year or so we added these capabilities. Full packet capture enabled development of the transcript tool (we also added string matching in the data portion of the packets), which proved invaluable for operations (and were inspired by Cliff Stoll's work). And we integrated with a host-based monitor in the DIDS system. A year later we started distributing the NSM.